Polyvinyl alcohol is a thermoplastic polymer material having excellent gas barrier property and transparency. This material, which has a relatively low glass transition temperature of about 80° C., is effective in thermoforming, and thus is widely used as raw materials of films, sheets, containers, etc. Moreover, the material coated with another resin film or sheet is often used to improve oil resistance and gas barrier property. Unfortunately, polyvinyl alcohol has poor solubility with respect to common organic solvents and is very difficult to be handled as a liquid material. Additionally, polyvinyl alcohol has low reactivity with a modifying agent for imparting additional functions. Thus, this material is difficult to be modified, and the use thereof is limited. Accordingly, it is desired to develop polyvinyl alcohol having improved solvent solubility and excellent handleability as a liquid material while keeping characteristics of polyvinyl alcohol.
To solve the problems, Patent Document 1 proposes a polymer having an alkoxy group at side chains. This polymer can improve the solubility. However, the alkoxy group bonded to Si at side chains has strong hydrolyzability, and another problem of solution stability arises. Moreover, there is no consideration for solubility with respect to silicone solvents and aliphatic hydrocarbon solvents having high safety.
Patent Document 2 proposes a polymer having a linear siloxane at side chains. This polymer can improve solubility with respect to aromatic hydrocarbon solvents such as toluene and xylene and polar solvents such as N,N-dimethyl formamide (DMF) and methyl ethyl ketone (MEK), but has poor solubility with respect to silicone solvents and aliphatic hydrocarbon solvents having high safety. Thus, the above problems are not fundamentally solved.
Patent Document 3 provides a substituted silylalkyl carbamate polyvinyl alcohol as a material that has general characteristics of polyvinyl alcohol such as film-forming property, toughness, excellent gas barrier property, and transparency as well as characteristics of silicone having a branched structure such as high solubility with respect to an organic solvent and excellent handleability as a liquid material. However, since a usual polyvinyl alcohol has low reactivity with a silicone modifying agent, an excessive amount of the silicone modifying agent is required to obtain the substituted silylalkyl carbamate polyvinyl alcohol with high modification rate, increasing the production cost. Thus, this material is unsuited for an intended use in many cases.